Hans Dampf wrote:
> How can I define, which c-compiler should be used installing gems with
> c-code on windows?
> Is there a possibility to select a compiler (mingw, bcc32, cygwin) or
> does it always use msvc?

By default it's the same compiler that you used to compile Ruby. 
Otherwise you could try setting the CC environment variable.

set CC=bcc32

-- 
RMagick: http://rmagick.rubyforge.org/